The present invention relates generally to processing and displaying digital data and more specifically to processing digitally modulated RF signals for presenting measurement data related to the peak-to-average power of a digitally modulated RF signal, such as 8-VSB digital television signals.
The Federal Communications Commission has adopted the Digital Television Standard developed by the Advanced Television Systems Committee (ATSC). The Digital Television Standard is designed to transmit high quality video and audio and ancillary data over a 6 MHZ channel. The system delivers reliably over 19 Mbps of throughput in a 6 MHZ terrestrial broadcasting channel and about 38 Mbps of throughput in a 6 MHZ cable television channel where a higher signal to noise ratio is ensured. The Standard describes the channel coding and modulation RF/transmission subsystems for terrestrial and cable applications. The modulation uses a digital data stream to modulate the transmitted signal. The modulation subsystem offers two modes: a terrestrial broadcast mode (8-VSB), and a higher data rate mode (16-VSB).
The modulation technique implemented in the Digital Television Standard employs vestigial sideband modulation that was developed by Zenith Electronics Corporation. The overall system response of the combined transmitter and receiver utilizes a raised cosine filter to eliminate inter-symbol interference. The system response is implemented with matched root raised cosine filters in the transmitter and in the receiver. The incoming digital data stream is randomized, forward error corrected (FEC) and interleaved. The randomized, FEC coded and interleaved data is trellis encoded as an 8-level (3-bit) one dimensional constellation. The outputs of the trellis coder are referred to as symbols that are one of eight discrete odd level integers from -7 to +7 set by the encoder. To aid synchronization in low signal to noise and high multipath situations, segment and field syncs are added to the 10.76 Msymbols/sec signal as well as a small pilot tone at the carrier frequency generated by offsetting the real or I channel of the composite signal containing the data and the sync pulses by 1.25 units. At the transmitter, the composite signal passes through a root raised cosine filter and modulates an intermediate frequency carrier signal which is up-converted to an RF frequency for transmission at the desired channel frequency. The offset causes the pilot tone to be in-phase with the I channel carrier frequency. Alternately, the composite signal may directly modulate the RF carrier.
In the Digital Television Standard, the average power of the digital TV signal is independent of the scene content, motion, and other variables and is suitable for measuring. However, because of the nature of the digital modulation scheme, transient peaks exist in the transmitted signal that are random in nature and need to be expressed statistically in terms of percentage of time in which the transient exceeds the average power by a stated value expressed in dB. Through measurements on experimental digital transmission systems, the optimum ratio of the transient peak power to the average peak power is stated as being approximately 6 dB. With 6 dB of headroom between the average power and the peak power, transient peaks will be encountered in a range of about 0.24% of the time. These transient peaks are important for non-linear considerations, such as compression of the output devices and voltage breakdown in transmission lines.
A paper titled "Measuring Peak/Average Power Ratio of the Zenith/AT&T DSC-HDTV Signal with a Vector Analyzer" by Gary Sgrignoli appearing in the IEEE Transactions of Broadcasting, Vol. 39, No.2, June 1993 describes a Cumulative Distribution Function (CDF) of peak-to-average power ratio for digital television signals and the requirements and process for making this measurement using a Hewlett-Packard 89440A Vector Signal Analyzer. The HP 89440A Vector Signal Analyzer was used to acquire and processes the HDTV signal to produce a time versus voltage output display that is interpretable as a CDF of peak-to-average power ratio display. Because CDF displays are not a resident function in the vector analyzer, there was a requirement for programming the vector analyzer to acquire and display the relevant data using an Instrument BASIC program language built into the instrument that includes a resident editor, debugger and programming utilities. The instrument was programmed to initialize the instrument to power-on default parameters and provide a prompt for entering the center frequency of the modulated RF or IF data signal and to specify the number of captured data blocks ("runs") to process. The average power of the HDTV power envelope was determined by using power band markers set to a 6.0 MHz bandwidth centered around the center frequency. A long term measurement was performed in which the signal was RMS-averaged for 100 data blocks. Time samples of the signal envelope power samples were captured over a data block of 2048 sample points and a peak-to-average power histogram was created. The instrument looped back and acquired another block of envelope time samples in a single sweep mode until the initially specified number of runs was completed. The author indicated that 100 data blocks were used for acquiring and displaying the data.
A CDF display was created by scaling and integrating the peak-to-average histogram. The x-axis of the display was labeled as time in .mu.sec instead of peak-to-average power ratio in dB, and the y-axis was labeled as voltage in V.sub.pk instead of percentage. This was the result of using the vector signal analyzer screen to display nonstandard output data. The author indicated that the displayed data could be more accurately displayed by converting the CDF plot to a Y-axis log plot by transferring the data to a floppy disk and using external plotting software. The CDF plot was converted to ASCII format on a PC-compatible computer using Hewlett-Packard utilities software. The data was then read into a stand-alone graphics software package for subsequent log plotting. The time versus voltage plot on the Vector Signal Analyzer could be interpreted by reading the Y-axis marker directly as percent and not as V.sub.pk and reading the X-axis marker value directly as dB and not as .mu.sec and subtracting 30.
A later version of the Hewlett-Packard vector signal analyzer, the HP89441A, provides a user with a selectable peak percentage value and reads out a peak-to-average power ratio in dB without providing a display of the CDF of peak-to-average power ratio. Using buttons on the front panel of the instrument, the user calls up a menu for selecting a peak percentage of time values between 90 and 99.9%. The instrument acquires and processes the IF signal representative of the HDTV RF signal and reads out the peak-to-average power ratio for the selected peak percentage of time value.
The use of the above described instruments requires a technical knowledge by the user in 8-VSB technology. This knowledge would include an ability to relate the CDF plot of peak-to-average power ratio to an ideal CDF plot of peak-to-average power ratio and use that knowledge in relation to an "operating point" or "planning factor" for setting up the 8-VSB transmitter. The user would need knowledge of what an ideal CDF plot of peak-to-average power ratio looks like and the x and y coordinates for the various points on the ideal CDF curve. The user would also have to have an ability to interpret the time versus voltage display into the CDF plot of peak-to-average power ratio. It also requires a user to download data from the instrument to a PC and perform additional processing to accurately generate a CDF plot of peak-to-average power ratio. To generate a CDF plot of peak-to-average power ratio using the later version instrument would require manually selecting a series of peak percentage values from the available peak percentage values and manually plotting on paper the resulting peak-to-average power ratio values.
What is needed is a measurement acquisition and display system for generating and displaying a graphical representation of a CDF of peak-to-average power ratio of a digitally modulated RF signal, such as the 8-VSB terrestrial broadcasting signal, along with an ideal CDF of peak-to-average power ratio graphical representation of the 8-VSB terrestrial broadcasting signal for comparing with the real peak-to-average power ratio measurement. The measurement acquisition and display system should allow a user to enter planning factor information and receive data back based on the acquired and displayed CDF of peak-to-average power ratio measurement. In addition, the measurement acquisition and display system should access the total range of peak-to-average power ratio values and percent of time values associated with the CDF of peak-to-average power ratio measurement. Further, the measurement acquisition and display system should provide continuous monitoring and alarm capabilities.